A demand for high repetitive operation recently increases in a glass laser. Phosphate laser glass has a narrow spectral width and is very suitable for such a use. If, however, the high repetitive operation is carried out, heat from a flash lamp as a pumping light source is accumulated in the glass, resulting in increasing the temperature of the glass. To lower the temperature of the glass, therefore, water cooling or air cooling is usually applied. However, since the thermal conductivity of glass is small as compared with that of crystals, even if water cooling, for example, is applied, the heat accumulated in the inside of the glass is difficult to diffuse to the cooling medium. As a result, a large temperature distribution tends to occur in the glass. As a result, the glass is broken by thermal stress due to the temperature distribution. In order to prevent the breakage of glass, it is necessary to increase the thermal shock resistance of the glass.
The thermal shock resistance R is represented by ##EQU1## wherein .mu. is a Poisson ratio, .kappa. is a thermal conductivity, S is a rupture strength (bending strength), .alpha. is a coefficient of thermal expansion and E is a Young's modulus.
The factors .kappa., .mu., .alpha. and E are constants for a given substance. It can be seen, therefore, that to increase the thermal shock resistance of glass having a given composition, it is sufficient to increase the rupture strength of the glass.
Several techniques have been proposed to increase the thermal shock resistance of glass. A typical example is a chemical strengthening method of the surface of glass by ion exchange. This chemical strengthening method, however, can be applied only to specifically formulated glass compositions and cannot be applied to other glass compositions such as a phosphate laser glass composition formulated in view of the high repetitive operation characteristics. Another problem of the chemical strengthening method is that the compression stress layer formed on the surface of glass by the ion exchange process, when the temperature of the glass rises, disappears due to migration of ions.